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electrical issue - need help


rossman

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So, the problem is when I go to connect the negative terminal on my battery it arcs every time.  Big spark, lead spattering off the battery terminal, ignition off, key out.  I assume this means I have a short somewhere. Here is a little history.  It's a 1972 240z with extensive electrical modifications, build page here.  The car was running, no arcing, no electrical issues.  I pulled the dash to install an aftermarket speedo and tach, neither of which are connected to power at the moment. I verified that all the passenger side connections have the same color wires on both sides of the connectors.  I pulled all connectors on to the stock fuse box and disconnected power to it.  I haven't messed with anything under the hood except removing the old battery and reinstalling a new one. One thing I noticed is that some of the under-dash wire connectors are not obvious where they connect.  I'm worried I have a ground connector connected to power.

 

I'm looking for tips to track down the short, like where I should start and how to proceed and if there are any common culprits.  

 

Thanks for any help!

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Use a meter and check for shorts from the positive cable connections to ground.  The starter lug that the positive cable is connected  to and the alternator charge wire which is usually connected to the same spot.  One of those two has to be grounded.  Then narrow it down from there.  If it's the starter, easy, if ti's the charge wire you'll have to check the voltage regulator and the various loads along the way.  All you can do is keep checking with the meter until you find that short circuit.  Sounds like a big one though, and since the fusible link doesn't blow I'm guessing starter.

 

Then there's always the reversed connections.  Happens to many.

 

http://www.atlanticz.ca/zclub/techtips/wiringdiagrams/240z/1972_240z.gif

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NewZed - Thanks for your reply.  Essentially the only thing stock about the electrical system is the lighting wiring. I've replaced all the fusible links with maxi fuses.

 

I put my DMM on the 10A setting then put the terminals inline with the negative battery terminal and the DMM internal fuse popped immediately.  Luckily I have a spare! How can I test for the short without making an electrical connection to the battery?  Continuity test?  I get 10 - 30 milliohm resistance between ground and the alternator/battery positive terminal.  Alternator ground to other grounds have no measurable resistance; which makes sense.

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By "shorts" I meant continuity.  There shouldn't be any continuity from the end of the positive cable to the end of the negative cable (ground), when the key and everything else is off.  You don't even need the battery installed to do the testing.  Set the meter for ohms/resistance.

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By "shorts" I meant continuity.  There shouldn't be any continuity from the end of the positive cable to the end of the negative cable (ground), when the key and everything else is off.  You don't even need the battery installed to do the testing.  Set the meter for ohms/resistance.

 

Good idea. Leave the meter hooked up and go back to the wires you most recently played with and unplug the connectors one at a time. Place the meter where you can see it as you unplug connectors/wires. Just be methodical and do one thing at at a time.

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I didn't want to complicate the process but it could be a relay powered circuit that's shorting.  That's harder to find, since the relay needs power to cause the short.  Your description of molten lead makes it sounds like you have a big wire shorting though.  Looks like a tough one.

 

One thing you can do is to put a light bulb, like a bulb in a tail light socket, in between the end of the positive cable and the battery positive.  The light bulb will allow enough power to flow to cause the short, but not enough to burn anything up.  If the short is there the light will light up.  If it's a relay, you'll hear the relay click when you connect the power.  I think that you'll get enough current through a small bulb to activate a relay.  Even if it's not a relay the light will tell you if the short is there.  One possible way to figure it out.

Edited by NewZed
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